Semiconductor nanocrystals (NCs) or quantum dots (QDs) have been extensively investigated over the last decade for a variety of biomedical, biochemical sensing, and optoelectronic applications. An area that has received relatively little attention so far is the use of NCs as gamma or X-ray detectors in applications such as positron emission tomography (PET), digital radiography, dosimetry, and nuclear medicine. In a typical radiation detection system, conversion of the incident energy of ionizing radiation is accomplished by using scintillating materials that emit photons in the visible/UV spectral range, subsequently collected by a photosensitive element.
Positron Emission Tomography:
Positron emission tomography (PET) is an imaging technique for tracking the bodily uptake of positron-emitting isotopes in two or three dimensions. The technique is currently used in medicine for the detection and analysis of cancerous tumors [Bangerter 1998], Alzheimer's disease [Matsunari 2007], and epilepsy [O'Brien 2001]. The first step in performing a PET scan is the synthesis of a positron-emitting material. A commonly used material is 2-[18F]-fluoro-2-deoxy-d-glucose (18F-FDG, FDG), where a hydroxyl group from glucose is replaced with fluorine-18, a synthetic positron-emitting isotope. FDG is absorbed by organs of the body as glucose, and is concentrated in high activity tissues in the body such as tumors and the brain.
Fluorine-18, which has a half-life of 109.77 minutes, emits a positron and decays into stable oxygen-18. The emitted positron travels a few millimeters in tissue before an electron annihilates it, and the product of this interaction is a pair of 0.511 MeV gamma rays traveling in opposite directions due to conservation of momentum [Knoll 2000]. The detection of these pairs of gamma rays is accomplished through a coincident scintillation event in a pair of detectors on opposite sides of the body.
When an event is detected, it signals that a positron-electron annihilation has occurred in a volume defined by the two detectors, and through statistical evaluation of many decays, tomographic reconstruction provides a two-dimensional or volumetric image of the locations of the events. In current systems, thousands of scintillation detectors are arranged around the patient, allowing for coincident detection of the two gamma rays resulting from positron-electron annihilation to minimize this volume, approaching the ray integral required for image reconstruction [Kalk 1998].
Potential Advantages of Nanocrystals in Time-of-flight PET:
The signal-to-noise ratio of a tomographic image can be improved through time-of-flight methods [Wong 1983]. High-brightness high-speed scintillators, such as LYSO [Muzic 2006], allow for extraction of very small, on the order of picoseconds, differences in scintillation times to provide a spread function along the detection volume containing the location of the annihilation events. This additional information leads to a higher signal to noise ratio, lower doses of radioactive tracers, faster imaging, and increased resolution of the reconstructed image [Wong 1983]. Fast and efficient scintillators are therefore crucial for time-of-flight PET applications.
Commonly used extrinsic inorganic scintillators consist of a transparent insulator and an impurity functioning as a luminescence center. They are, in many cases, either slow or have low radiative efficiency. Indeed, in developing ultrafast scintillators, the luminescence via an intermediate excited state of an impurity is rather disadvantageous. Currently, the conventional cerium (Ce3+)-activated inorganic scintillators provide the best combination of speed and efficiency. Cerium-doped bulk lanthanum halide compounds are attractive due to their interesting properties that include efficient radiation absorption, highly luminescent activator (cerium), activators protected by the host material from luminescence quenching, and fast decay time of radiative transitions between internal atomic levels of cerium (10-60 ns). Among a variety of Ce-doped bulk crystals that possess good scintillation detection properties, Ce-doped bulk LaBr3 was reported to have a light yield of 60,000 photons/MeV, 2.5% full-width-to-half-maximum energy resolution for 662 keV γ rays, and 25 ns short decay time [Derenzo 2005]. Ce-doped bulk LuI3 was reported to have a record high light yield of 95,000 photons/MeV, 3.3% full-width-to-half-maximum energy resolution for 662 keV γ rays, and 24 ns scintillation decay time for the dominant decay component [Kramer 2006]. Recently, very impressive scintillation properties have been reported for (Eu2+)-activated alkaline earth halide SrI2 and BaI2 bulk materials [Cherepy 2008]. However, while having those attractive properties, LuI3:Ce, LaBr3:Ce, SrI2:Eu, and BaI2:Eu bulk crystals are expensive and difficult to manufacture and use. The crystals are highly hygroscopic or even deliquescent and very fragile, and hence have to be protected from external environment both during growth and use.
The radiative decay times of the conventional cerium (Ce3+)-activated inorganic scintillators are limited to ˜10-60 ns [van Loef 2001], [Weber 2002]. Direct excitonic luminescence from pure semiconductors in intrinsic inorganic scintillators could be employed by using the direct recombination of an electron and a hole with a decay time constant shorter than 10 ns. However, undoped semiconductors have rarely been used as scintillators, because of their poor luminescence efficiency at room temperature (RT). The excitonic level in a semiconductor is below the bottom of the conduction band by the binding energy Eb of the exciton. In bulk semiconductors, the excitonic level is not deep enough to prevent the thermal dissociation of excitons, and, as a result, the significant thermal quenching of excitonic luminescence at RT. Recently, very fast and efficient performance has been demonstrated from pure semiconducting scintillators such as PbI2 and HgI2 at cryogenic temperatures [Klintenberg 2002], [Derenzo 2002], [Klintenberg 2003]. Cooling the system to very low temperatures increased the population of excitons rather than free carriers by effectively suppressing the thermal perturbations, proportional to the thermal energy kBT.
Increasing the binding energy Eb of the exciton to the values exceeding the thermal energy kBT at RT (˜26 meV) is the requirement to thermally stabilize the excitonic level at RT. This can be realized by employing quantum confinement effect observed in low-dimensional quantum confinement systems (LD QCS). Enhancement of Coulomb interaction between the electron and hole due to spatial confinement is known to deepen the excitonic level in a low-dimensional system. For example, the binding energy of the lowest exciton confined in a two-dimensional (2D, quantum well) system is four times higher than that of the free exciton in the corresponding 3D bulk system [Papavassiliou 1997]. In addition to providing improved thermal stability of the excitonic population, quantum confinement affects the excitonic radiative and nonradiative lifetimes in a way that would further enhance the radiative efficiency. Due to much better overlapping of the electron and hole wavefunctions in a LD QCS, the excitonic oscillator strength increases and the excitonic radiative lifetime shortens [Amand 1992], [Xu 1993]. At the same time, the nonradiative lifetime lengthens due to a decrease in the effective density of nonradiative centers that can be encountered by the spatially confined excitons.
Nanoscintillators:
In contrast to ample literature on scintillators based on large-size crystals, there have been only a handful of reports on radiation response of colloidal NCs. The term “scintillation” is sometimes, perhaps confusedly, used to indicate wavelength conversion from UV to the visible [Mutlugun 2007]. Here, we consider scintillation to mean optical response (visible or UV) to ionizing radiation.
The first demonstration of NCs as scintillators for radiation detection was reported in [Dai 2002], where CdSe/ZnS core/shell colloidal QDs were used. The QDs were rendered water soluble by exchanging the surface ligands with dithiothreitol (DTT), added during the preparation of lithiated 6LiOH gels, and embedded in a transparent sol-gel matrix. Using a standard setup with a photomultiplier tube (PMT), amplifier, and a multichannel board, scintillation was observed under α irradiation from a 210Po source.
Commercial CdSe/ZnS colloidal QDs suspended in toluene were used in [Létant 2006a]. The QDs were inserted in porous glass with pores increased to 10-20 nm in diameter in order to increase their concentration. Scintillation from 1/16 in. thick nanocomposite was observed under α irradiation from 0.2 μCi 243-244Cm source. Due to a poor match between QD emission at 540 nm and a PMT used to record scintillation event, only 0.4% of photons emitted by the QDs were amplified, resulting in a poor, barely detectable signal. These results were significantly improved in a subsequent paper by the same authors [Létant 2006b], where a PMT with 15% quantum efficiency at 510 nm produced a clear pulse height spectrum, significantly above the background.
Nanoporous glass impregnated with CdSe/ZnS colloidal QDs emitting at 510 nm was also used to detect radiation from 1 μCi 241Am source, emitting 59 keV γ rays [Létant 2006b]. Energy resolution of ˜15% obtained by recording the scintillation output from 1 in. thick nanocomposite over the period of 3 days was twice better than the corresponding energy resolution of ˜30% observed for 1 in.×1 in. Ø bulk NaI:Tl crystal.
X-ray luminescence of BaFBr NCs doped with Eu or Mn, and of LaF3:Ce was studied in [Chen 2006]. BaFBr:Eu,Mn exhibited persistent luminescence (afterglow) for as long as 8 minutes after the X-ray excitation source was turned off, hence it is not suitable for fast radiation detectors.
Preliminary data on scintillation response of LaF3:Ce NCs embedded in an organic matrix and exposed to 241Am as well as 57Co (89% 122 keV and 11% 136 keV photons) sources were given in [McKigney 2007]. The energy resolution was stated as “not good”, supposedly due to low quantum efficiency of the LaF3:Ce NCs.
Compared to currently used scintillating particles of the micrometer size, NCs offer the prospect of significantly improved performance. Due to their small size, they may have better solubility in organic polymer or inorganic sol-gel host materials and to cause much less scattering, which should result in higher efficiency of the scintillator. While the bulk materials may have poor efficiency of light emission at room temperature, the effects of quantum confinement are expected to greatly enhance the probability of radiative transitions. Due to three-dimensional confinement and much better overlap of electron and hole wavefunctions, the optical transitions may be much faster than in bulk scintillators, which should eliminate the major problem of relatively slow response of scintillator detectors. In addition, NC scintillator material allows for scalability, ruggedness, and enhanced design flexibility, in general, of the entire detection system. Heterogeneous core/shell nanoparticle morphology can very effectively address the problem of stabilizing highly hygroscopic or even deliquescent scintillating material.
US publication Nos. 2008/0128624; 2008/0093557; and 2008/0191168 describe LaBr3:Ce nanoparticles in a matrix to provide a nanocomposite scintillator. However, stable non-hygroscopic shells for stabilization of highly hygroscopic LaBr3:Ce nanoparticles are not disclosed.